LEC 02.10 Boiling point elevation - Phywe

Related concepts 

Raoult’s law, Henry’s law, ebullioscopic constants, chemical 

potential, Gibbs-Helmholtz equation, concentration ratio, degree 

of dissociation. 

Principle 

The boiling point of a solution is always higher than that of the 

pure solvent. The dependence of the temperature difference 

(elevated boiling point) on the concentration of the solute can be 

determined using a suitable apparatus. 

Tasks 

1. Measure the increase in the boiling point of water as a function 

of the concentration of table salt, urea and hydroquinone. 

2. Investigate the relationship between the increase in boiling 

point and the number of pellets. 

3. Determine the molar mass of the solute from the relationship 

between the increase in boiling point and the concentration. 

Fig. 1. Experimental set-up. 

Boiling point elevation 

LEC 

02.10 

Equipment 

Apparatus for elevation of boiling point 36820.00 1 

Temperature meter, digital, 4-2 13617.93 1 

Temperature probe, Pt 100, Teflon 11759.04 1 

Heating hood, 250 ml 47550.93 1 

Clamp for heating hood 47557.01 1 

Power regulator 32247.93 1 

Flask, round, 250 ml, GL 25/12 35812.15 1 

Glass beaker, 250 ml, tall 36004.00 1 

Jointing, GL 25/8 41242.03 1 

Silicone tubing, d i = 7 mm 39296.00 1 

Retort stand, h = 750 mm 37694.00 1 

Right angle clamp 37697.00 3 

Universal clamp 37715.00 3 

Mortar with pestle, 190 ml 32604.00 3 

Pinchcock, w = 15 mm 43631.15 1 

Microspoon 33393.00 1 

Pellet press for calorimeter 04403.04 1 

Precision balance, 620 g 48852.93 1 

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Weighing dishes, 80 x 50 x 14 mm 45019.05 1 

Funnel, d o = 80 mm 34459.00 1 

Pasteur pipettes 36590.00 1 

Rubber bulbs 39275.03 1 

Wash bottle, 500 ml 33931.00 1 

Beads, 200 g 36937.20 1 

Sodium chloride, 500 g 30155.50 1 

Urea, pure, 250 g 30086.25 1 

Hydroquinone, 250 g 30089.25 1 

Glycerine, 250 ml 30084.25 1 

Water, distilled, 5 l 31246.81 1 

Vice 

Set-up and procedure 

Set up the experiment as shown in Fig. 1. 

Weigh the dry inner vessel of the boiling point apparatus and 

note the exact mass (= m 1 ). Fit the inner vessel into the outer 

vessel so that its inlet opening is located below the silicone rubber 

seal of the connecting cap. During measurement, steam is to 

enter the inner vessel through the lateral aperture, so pay attention 

that it is not covered. Fill the round flask with 150 to 200 ml 

of water and connect it to the assembled apparatus. Slip two 

short pieces of silicone tubing onto the two gas outlets of the 

outer vessel and place the lengths in a 250 ml glass beaker with 

the free ends at about the middle of the beaker. Attach a pinchclip 

to the lower of the two tubes coming from the outer vessel, 

but for the time being leave it open. Pour approximately 40 ml of 

water into the inner vessel. Close the vessel at the top by fixing 

the temperature probe in position. 

The substances to be tested must be pressed into pellets in 

order to prevent any particles of them from sticking to the walls 

of the vessel while being added. Weigh out five portions of each 

substance (NaCl, urea, hydroquinone), each of approximately 

700 mg. It is advisable to first pulverise the substances with a 

mortar and pestle. Use the pellet press as follows: Place it in a 

vertical position and put the small steel rod in the cylinder to 

close the bottom end of the borehole. Fill one portion of the substance 

into the hole using a funnel. Next insert the large rod from 

2 



Fig. 2: Example of a measurement: boiling point crease as 

function of concentration of table sal an aqueous solution 

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above and compress the substance a little. Fit the assembled 

press in a vice and apply pressure on it, so that a solid pellet is 

formed from the substance. Press the pellet out of the borehole 

with the longer rod. 

Weigh the pellets to an accuracy of 1 mg. The dependence of 

the boiling point elevation of water on the concentration can be 

demonstrated for each substance in a single experiment by successively 

adding known amounts of the same substance. Heat 

the solvent in the flask to boiling. The vapour evolved rises up 

into the outer vessel and heats up the inner vessel. Control the 

heating rate with the power control. The temperature in the inner 

vessel is displayed on the digital temperature meter in degrees 

Celsius. After some minutes, when the temperature in the inner 

vessel has nearly reached the boiling point and no longer 

increases, lower the heating hood for a few seconds until boiling 

stops and the condensate on the outer vessel returns to the 

round flask. Then raise the heating hood again. When boiling 

recommences, close the pinchcock. The (slightly superheated) 

steam now flows through the water in the inner vessel. Set the 

digital temperature meter to measure the change in temperature 

∆T over time with the tare function . In this mode of 

operation, the resolution is tenfold better (0.01 K). Wait until the 

value displayed remains constant. Now carefully open the inner 

vessel (screw cap), add the first substance pellet, and close the 

opening immediately. The temperature first drops slightly and 

then rises again while the pellet dissolves. When the value has 

again become constant, record it and repeat this procedure for 

the next portion of the substance. After five concentration steps 

have been measured, first open the pinchcock and then switch 

off the heating. This is important to avoid solution being sucked 

from the inner vessel into the flask containing water while cooling 

down. Remove the inner vessel, dry its outer surface, remove 

the temperature probe and re-weigh it. The mass of the water is 

now equal to the last measured value less the mass of the empty 

vessel and the masses of the five substance pellets. Plot the 

increase in boiling point against the quotient of the mass of the 

dissolved substance and the mass of water for each substance 

as shown in Fig. 2. 

Theory and evaluation 

A solution is a mixed-phase liquid which consists of a dissolved 

substance and a solvent. Only the solvent is capable of vaporization, 

the vapour pressure of the solute is practically zero. 

When a substance dissolves in a solvent, additional forces result 

and these must be overcome by solvent molecules before they 

can pass into the gas phase. Less solvent molecules can therefore 

vaporize from a solution than from the pure solvent. In 1886, 

F. M. Raoult postulated the law that is named after him: The 

vapour pressure of a solution is given by the product of the 

vapour pressure of the pure solvent and the mole fraction of the 

solvent. 

where: 

ps p0 n1 n2 n 2 

Ps · p0 n1 n2 Vapour pressure of the solution 

Vapour pressure of the pure solvent 

Amount of the pure solvent 

Amount of dissolved substance 

A liquid boils when its vapour pressure is the same as the ambient 

pressure. The vapour pressure of water reaches an ambient 

pressure of 1013 hPa at a temperature of 100°C. When a substance 

is dissolved in water, the vapour pressure is reduced, and 

is so less than 1013 hPa at 100°C. Heat must now be supplied 

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to increase the kinetic energy of the molecules, and so to raise 

the temperature so that the solution comes to boiling. The reason 

for the higher energy requirement for the solution than for 

the pure solvent is because additional forces, mutual attractive 

forces between solute and solvent, must be overcome in the 

solution. The solution does not boil at T 0 , but at the higher temperature 

T s . The difference between these two temperatures is 

the boiling point elevation ∆T s . 

∆T s = T s - T 0 (2) 

From a quantitative point of view, the boiling point elevation is 

dependent on the amount to which the vapour pressure is lowered, 

and so on the concentration of the solute. Molality is used 

here as dimension, i.e. the number of moles of solute dissolved 

in 1 litre of solvent. 

m1 m2 M2 Ke ∆T K e · m 2 · 1000 

M 2 m 1 

Mass of the pure solvent 

Mass of the dissolved substance 

Molar mass of the dissolved substance 

Ebullioscopic constant 


(3) 

When determining the relative molar mass of a dissolved substance, 

the fact that the number of free moles n B corresponds to 

the number of free particles must be taken into account. If, however, 

n B moles dissociate into z smaller particles in solution, then 

the number of moles actually present becomes 

n = n B (1 + (z – 1) a) (4) 

where a is the degree of dissociation. 

Under certain conditions, the determination of molar masses 

using ebullioscopy can therefore only supply the apparent molar 

mass M s , which must then be converted using 

M s 

Data and results 

K e (water) = 0.515 K · kg · mol -1 

M B 

11 1z 12 a2 

Molar masses (in g · mol -1 ): 

NaCl (a = 1, z = 2): 61.71 (exp.); 58.44 (lit.) 

Urea (a = 0): 60.83 (exp.); 60.06 (lit.) 

Hydroquinone (a = 0): 108.14 (exp.); 110.11 (lit.) 



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PHYWE series of publications • Laboratory Experiments • Chemistry • © PHYWE SYSTEME GMBH & Co. KG • D-37070 Göttingen

LEC 02.10 Boiling point elevation - Phywe

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